@B: lightblue;

html,
body {
  height: 100%;
//   background-color: @B;
  overflow: hidden;
}

* {
    margin: 0;
    padding: 0;
}
.center {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}
#root {
    width: 300px;
    height: 300px;
    background: lightblue;
}
.outer {
    .center;
    width: 200px;
    height: 200px;
    background: lightgreen;
}
.inner {
    .center;
    width: 100px;
    height: 100px;
    background: lightcoral;
}